For various reasons, it is often necessary to control, inspect or check the surface state of one or more faces of a solid and this is more particularly the case with laminated nuclear fuel.
One variety of nuclear fuel for experimental reactors is constituted by aluminium sandwich plates with a core made from a mixture of uranium and aluminium. The production process of said plates consists of laminating together four components, namely a compacted aluminium and uranium core mounted in a frame and covered by two plates forming a cover.
Although highly industrial, this production process requires several manual operations and know-how, so that there are variations in the quality of manufacture. However, as these products are particularly sensitive, they require very precise characteristics imposed by the users. These requirements concerning quality give rise to internal and external controls of the fuel plates.
The internal control takes place by special machines using ultrasonics or X-rays. The control takes place automatically and gives rise to reports combining quantitative and calibrated measurements.
The external control has hitherto been performed visually by qualified operators. The procedure involves observing two surfaces of a plate under glancing light, whilst manipulating the plate and locating or pinpointing surface defects. The latter are holes or scratches, whose depth must not exceed 100 .mu.m. When a surface defect appears suspect to the operator, he places the plate beneath the objective of a microscope and evaluates the maximum depth of the hole or scratch previously located. Plates having characteristics falling outside the standard are disposed of as waste. Certain users have particularly strict requirements, which requires a supplementary control performed by another team.
This external control, which is entirely the responsibility of human evaluation, suffers from a number of disadvantages. It is fastidious and tiring due to the concentration and acuity required. Its reliability is dependent on the state of vigilance, which varies in time and between individual operators. In the case of a second control, when the quality tends towards 100%, it remains virtually impossible to maintain the necessary vigilance for detecting the very rare defect or objectively impose acceptance limits. In addition, visual measurements offer no quantitative and objective basis for the support of a control or inspection report.
The invention makes it possible to improve the control of surface states and is applicable both to equipment used by an operator and to an automatic control installation. The observation conditions are improved by photographing the surfaces to be controlled, said photographs either being presented on a video screen at a control station, or are digitally processed in the case of an automatic control installation. The use of an optoelectronic probe makes it possible to obtain objective and quantitative depth measurements, which can be recorded.